Harper and his chief rival, Liberal candidate Justin Trudeau, embody opposite visions of what Canada should be. After a decade in power, Harper is seeking a rare fourth term in the hopes of safeguarding his goal of shattering Canada's reputation as a liberal haven. But he is trailing in the polls behind Trudeau, the son of late Prime Minister Pierre Trudeau, a Liberal icon who opened Canada's door wide to immigration and is responsible for the country's version of the U.S. Bill of Rights. Former colleagues of Harper say a loss to Trudeau's son would be personally devastating to Harper, who entered politics to end Trudeau's liberal legacy.

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DIFFERENT VISIONS FOR CANADA

Harper favors smaller government, tight spending and is more hawkish on national security policies. He has nudged the traditionally center-left country to the right, lowering sales and corporate taxes, avoiding climate change legislation and supporting the oil industry against environmentalists. Trudeau promises to hike taxes on the rich and run deficits for three years to increase government spending and boost the economy. A third candidate, New Democratic Party leader Tom Mulcair, favors more government spending but says he'd balance the federal budget.

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RELATIONS WITH THE UNITED STATES

A Trudeau victory could mean improved relations between Canada and the United States, which have soured over President Barack Obama's reluctance to approve the Keystone XL pipeline. Trudeau supports the Alberta-to-Texas pipeline, which is crucial to Canada as it seeks infrastructure to export its growing oil sands production. But Trudeau says relations between the two major trading partners should not hinge on one project and has vowed to smooth over tensions. Harper has clashed with Obama on several issues, including the historic nuclear deal that the U.S. and other world powers recently reached with Iran.

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A MINORITY GOVERNMENT?

No party is likely to get a majority of Parliament's 338 seats. That means the party forming the government could have a shaky hold on power and need to rely on another party to pass legislation. If the Conservatives win the most seats, the Liberals and New Democrats say they'll band together to bring down Harper's government in a vote in Parliament, raising the possibility of a coalition government. If the Liberals win the most seats, they're expected to rely on the New Democrats for support on a bill-by-bill basis. Harper previously won two minority governments before breaking through with a majority in 2011.

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THE IMPORTANCE OF TORONTO AND ITS SUBURBS

With 58 seats at stake, Canada's most populous city and its surrounding suburbs will be critical to the outcome of the election. Harper won a majority in 2011 capitalizing on support from traditionally conservative western Canada, but also from suburban immigrants around Toronto. If the districts around Toronto go Liberal, it will be a long night for Harper.

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POLL CLOSINGS

The first polls, in Newfoundland, close at 7 p.m. EDT (2300 GMT) and the last ones, in the Pacific region, at 10 p.m. EDT (0200 GMT). Results are generally called between 10 p.m. and 11 p.m. (0200-0300 GMT) GMT). If Liberals get the most seats, Trudeau would become prime minister. If Harper gets the most seats but falls short of a majority, his stay in power could be temporary as Liberals and New Democrats would seek to replace him after parliament reconvenes, within weeks or months.